The present invention is concerned generally with an improved oximeter for non-invasively measuring arterial oxygen saturation. More particularly, this invention is concerned with an improved method for direct digital signal formation from input signals produced by a sensor device which is connected to the oximeter.
In all oximeters, input signals are received from a sensor device which is directly connected to the blood-carrying tissue of a patient, such as a finger or ear lobe. The sensor device generally consists of a red LED, an infrared LED, and one or two photodetectors. Light from each LED is transmitted through the tissue, and the photodetectors detect the amount of light which passes through the tissue. The detected light consists of two components for each bandwidth. An AC component represents the amount of pulsating blood detected, while the DC component represents the amount of non-pulsating blood. Therefore, four separate components of detected light are examined in order to determine the arterial oxygen saturation: red DC, red AC, infrared DC and infrared AC. The amount of light detected is then used to determine the oxygen saturation in the blood of the patient based on the following equation:
(IR(AC)/IR(DC))/(Red(AC)/Red(DC))
In a traditional oximeter, the sensor output signal is converted to an analog voltage and then separated into infrared and red components. Some oximeters further separate the AC and DC components. Separate analog circuits are then used to sample, demultiplex, and filter these signals. In these systems, therefore, it is necessary to carefully match the analog components to minimize errors which can result from differences in gain or frequency response in the two circuits. Furthermore, because of the need to carefully match hardware for each analog input circuit, and the increased probability of errors when additional analog channels are added, traditional oximeters are generally limited to two analog inputs.
Additionally, the analog circuitry employed in traditional oximeters is generally insufficient to accurately detect low level signals. Therefore, these oximeters are generally ineffective for monitoring fetal conditions, as well as for use with patients with thick or very dark skin. Furthermore, the methods used in prior art oximeters for measuring oxygenation levels rely heavily on pulse detection and peak-valley measurements which are highly susceptible to variations due to motion artifact noise.
The instant invention improves on the analog signal processing employed in prior art oximeters by receiving input current signals from at least two and preferably three light emitting devices of different wavelengths and converting these input signals directly to digital voltage values, without first converting to analog voltages or separating the signals. This is accomplished by using a charge digitizing analog to digital converter with sufficient range to represent the large DC signals and sufficient resolution to represent the small AC signals. This charge digitizing converter employs a current integrator as the front stage, which tends to average and filter input noise. This is an improvement over the analog current to voltage conversion used in traditional oximeters, which tend to amplify noise.
Once the input current is converted to a digital voltage value, all input signals are processed along the same digital hardware path, instead of the separate analog hardware paths required by the traditional method. This system eliminates the need to match analog hardware components, and therefore further reduces potential errors. Furthermore, once the signals are digitized, a microprocessor can perform all of the signal processing, demultiplexing, and filtering steps required by traditional oximeters. This reduction in the analog signal processing stage increases both the speed and accuracy of the oximeter, decreases cost by eliminating expensive analog components, and reduces the size of the oximeter by eliminating physically large analog components.
In another aspect of the invention, a method for analyzing oxygenation levels without the need for pulse detection and peak-valley measurements is also disclosed. The method comprises the steps of storing vectors of contiguous, paired infrared and red data samples over a period of time, using a least-squares minimization method for determining an infrared to red ratio, and determining a noise metric for filtering noise from the resultant oxygenation calculations. The noise metric substantially filters noise due to motion artifact, such as source: detector geometry variations and respiration noise, thereby providing a more accurate oxygenation level reading.
In a further improvement, additional wavelengths can be added to the oximeter to improve noise filtering or add medical monitoring functions to the oximeter. Because all signal conversion is time-multiplexed through a single analog to digital converter circuit, a third or further wavelengths can be easily and inexpensively added to the sensor and device. The additional wavelengths can be used in a number of applications which increase the accuracy of the oximeter or provide additional monitoring functions, including: noise detection; dyshemoglobin detection and/or measurement; and indicator dye measurement.
Due to the ability of the digital circuitry of the present invention to process low level current input signals and to filter noise components and the additional noise filtering functions disclosed, the oximeter can be used to accurately monitor oxygenation levels which were previously difficult to monitor, including fetal oxygenation levels and the oxygenation levels of dark and thick skinned patients. In one particular embodiment the dynamic range of the analog to digital converter may be optimized to match the input signal range.
It is therefore an object of this invention to provide an improved method for non-invasively measuring fluid parameters.
It is another object of this invention to provide an improved method for measuring arterial blood saturation.
It is another object of the invention to provide improved speed and accuracy in the measurements provided by oximeters.
It is another object of the invention to provide a direct analog to digital conversion of the input current signal with sufficient range to measure large DC signals and enough resolution to represent small AC signals so that accurate measurements can be made with reduced analog signal processing.
It is another object of the invention to provide a reduction in potential errors by directly converting the input current signal to a digital voltage signal, thereby bypassing the current to voltage conversion step which can amplify noise.
It is another object of the invention to provide a reduction in potential errors by processing all signals along one digital hardware path, thereby eliminating the need for matched analog components.
It is another object of the invention to provide an improved oximeter having a reduced number of electronic circuit components.
It is still another object of the invention to provide a reduction in the size of oximeters by eliminating physically large analog components.
It is yet a further object of the invention to provide an improved method and system for directly converting to digital signal form at least two signals from light emitting devices of different wavelengths.
It is another object of the invention to provide an improved method for filtering noise from oxygenation level calculations.
It is yet another object of the invention to provide a dynamic range control for calculating oxygenation levels in a plurality of signal range levels.
It is still another object of the invention to provide an improved oximeter capable of monitoring a wide range of patients.
It is another object of the invention to provide a reduction in the size and cost of detecting more than two wavelengths in oximeters.
These and other object and advantages of the invention, together with the organization and manner of operation thereof, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings described below.